1. Field of the Invention
The present invention relates to apparatus for influencing fluid flow over a surface, and more particularly, but not exclusively, to turbulent boundary layer flow drag reduction for an aircraft.
2. Discussion of Prior Art
The boundary layer is a thin layer of fluid (air) that forms, for example, on an aircraft wing during flight adjacent to the surface of the wing in which viscous forces exert an influence on the motion of the fluid and in which the transition between still air and the wing's velocity occurs. Boundary layer control techniques are known where the airflow in the boundary layer is modified to increase and/or decrease drag.
Turbulent boundary layer flow is not yet fully understood, but the recognition that coherent structures exist has allowed efforts to be directed to modifying and/or controlling the turbulent boundary layer flow, as described, for example, in AIM paper 96-0001—“Control of Turbulence”—J. Lumley. Direct numerical simulation of the turbulent boundary layer flow, as described, for example in Phys. Fluids A 4 (8)—“Suppression of Turbulence in Wall-bounded Flows by High Frequency Spanwise Oscillations”—W. J. Jung, N. Mangiavacchi, R. Akhavan shows that disrupting the coherent structures could have a dramatic effect on the skin friction, reducing it by up to 40%. If this level could be achieved on an aircraft this would equate to a reduction in total drag of between 10% and 20% offering substantial savings in fuel and/or increases in range. Experimental verification of this numerical prediction has been achieved and published in AIM paper 97-1795 —“Turbulent Boundary Layer Control by means of Spanwise-wall Oscillation”—K-S. Choi, P. E. Roach, J-R. DeBisschop, and B. R. Clayton. In that paper the use of mechanical oscillation is described to demonstrate skin friction reductions of up to 45%. However, the paper does not suggest how a practical mechanical oscillation system could be implemented successfully.
Other approaches for actively disrupting the coherent structures, such as blowing, or using tiny micro-electro-mechanical actuators have been postulated, but no practical and effective means have been demonstrated. Passive modification of the coherent flow structures has also been attempted, for example, riblets and large eddy break-up devices. These approaches have achieved skin friction drag reductions, but at a much smaller level (less than 10% as opposed to 40%) and are therefore marginal in their overall benefits once extra cost and other penalties (such as increased weight) are considered.
There remains a need for a passive or active system that disrupts the coherent turbulent boundary layer structures to achieve large skin friction reductions, which is both practical and cost effective. In the 1998 conference publication AIM 36th Aerospaces Meeting, paper AIM 98-0328—“Boundary Layer Control with a One Atmosphere Uniform Glow Discharge Surface Plasma”—Reece-Roth, Sherman and Wilkinson, an electrode system based on rigid printed circuit board material is described and the interaction of surface plasmas with boundary layers related. The electrode system comprises a single set of a multiplicity of parallel conductive lines all electrically connected to one another. The plasma generating circuit is a high voltage radio frequency source operated at 3.0 kHz. The interaction of the surface plasmas with the airflow was said in this paper to be due to an electrostatic attractive force—termed paraelectric. At the end of the paper it is postulated that this sort of technology might be applicable to the generation of span-wise oscillations for turbulent drag reduction. No information on how this concept could be achieved was given.
Both travelling wave and span-wise oscillation boundary layer disturbances have been tried for drag reduction in sea water (using a combination of magnetic and electric field forces). Travelling waves have been found to be more effective, at least under certain conditions, as described in Science 288, 1230 (2000)—“Suppressing Wall Turbulence by Means of a Transverse Traveling Wave”, Du and Karniadakis.